Diffusion of chemisorbed oxygen into Pd sub-surfaces and its influence in oxidation catalysis

Oxygen migration to subsurface layers of palladium and its influence in catalytic oxidation has been addressed in this mini review. Oxygen dissolution in palladium is a well-known phenomenon; many researchers observed the presence of surface, subsurface and bulk oxides by different methods on palladium single crystals as well as in powder catalysts. Opinion is divided on deciding the nature of palladium that is responsible for catalytic oxidation reactions. Nonetheless it is tilting towards oxidized palladium, which is likely to be responsible for the oxidation catalytic activity, and increasing numbers of evidences are available. Oxygen covered in the subsurface layers of palladium show extended activity regime to higher temperature and it has been demonstrated by molecular beam methods. CO oxidation studied on Pd surfaces with CO+O2 mixture displays oxidation activity up to 900 K, highlighting a significant increase in CO adsorption capacity on the above surfaces. In this mini review we highlight the oxygen diffusion into Pd-sub surfaces and its characterization by several methods. Further how the subsurface oxygen could influence the electronic structure and hence catalytic activity has been briefly discussed through CO oxidation reactions

On the mechanism for the reduction of nitrogen monoxide on Rh(111) single-crystal surfaces

Isothermal kinetic experiments were carried out with isotopically-labeled molecular beams in order to characterize the surface reactions involved in the reduction of nitrogen monoxide with carbon monoxide on Rh(111) single-crystal surfaces. The new data reported here offers support for the basic model advanced previously where N2 production takes place via the formation of N-NO intermediates at the periphery of atomic nitrogen surface islands. However, they also highlight a few additional subtle complications. In particular, the rapid desorption of a small amount of 14N14N upon switching from 14NO+CO to 15NO+CO mixtures points to the role of additional adsorbates nearby the reaction surface site in facilitating the dissociation of the N-NO intermediates. In addition, the results from experiments with mixed 14NO+15NO+CO indicate a combination of reactions at the edges of previously deposited 14N islands and the growth of new mixed 14N+15N surface clusters

Acid-base properties of Cu<SUB>1-x</SUB>Co<SUB>x</SUB>Fe<SUB>2</SUB>O<SUB>4</SUB> ferrospinels: FTIR investigations

Systematic IR spectroscopic studies were undertaken to investigate the acid-base properties of Cu-Co ferrospinels Cu1-xCoxFe2O4 (x=0 to 1) employed in phenol methylation to produce 2,6-xylenol. The IR spectra of the ferrospinels reveal that Fe3+ and Co2+ ions are mainly responsible for the various hydroxy groups on the surface. Temperature dependent IR studies of pyridine adsorbed on spinels and on the spinel phase with deliberately added metal oxide exemplify the contribution of the metal ions and their coordination state towards Lewis acidity. IR studies of the spinel surface with adsorbed CO2 and adsorption studies of electron acceptors such as 7,7,8,8-tetracyanoquinodimethane, 2,3,5,6-tetrachloro-1-4-benzoquinone and p-dinitrobenzene were carried out to evaluate the nature of the basic sites and the strength and distribution of the electron donor sites present on the spinel surface. It was found that the acidity (basicity) of the Cu1-xCoxFe2O4 spinel system increases (decreases) from x=0 to 1. A correlation between acidity, basicity and catalytic performance reveals that an intermediate acid-base character enhances the phenol methylation activity

Role of adsorbed nitrogen in the catalytic reduction of NO on rhodium surfaces

The role of surface nitrogen in the kinetics of the NO+CO conversion reaction on Rh(111) under steady-state catalytic conditions was explored by using collimated molecular beams and mass spectrometry detection. Two types of kinetically different nitrogen atoms were identified on the surface. The buildup of a critical nitrogen coverage was determined to be required for the start of the nitrogen recombination step to N<SUB>2</SUB>. This threshold coverage is quite large at low temperatures, amounting to over half a monolayer around 400 K, but decreases abruptly with increasing reaction temperature, and becomes almost insignificant above 600 K. The actual value of this coverage is quite insensitive to the ratio of NO to CO in the reaction mixture, but displays an inverse correlation with the steady-state reaction rate under most conditions. An additional small amount of nitrogen appears to be present on the surface during catalysis but to desorb rapidly after the removal of the gas-phase reactants. The NO reduction rate displays an approximately first-order dependence on the coverage of these labile N atoms. Isotope switching experiments indicated that the two types of kinetically different nitrogens are not likely to represent different adsorption sites, but rather similar adsorption states with adsorption energetics modified by their immediate surrounding environment on the surface. The data are explained here by a model in which the nitrogen atoms form surface islands and where the atoms at the perimeter of those islands react preferentially via N+N recombination to N<SUB>2</SUB>

A molecular beam study of the kinetics of the catalytic reduction of NO by CO on Rh(111) single-crystal surfaces

Steady-state rates for the catalytic reaction of NO with CO on Rh(111) surfaces have been measured under isothermal conditions by using a molecular beam approach with mass spectrometry detection. Systematic studies were carried out as a function of surface temperature, NO+CO beam composition, and total beam flux. A maximum in reaction rate was observed between 450 and 900 K, the exact temperature depending on the NO:CO beam ratio. Indeed, a synergistic behavior was seen where the loss in reactivity induced by increasing the CO concentration in the beam is partly compensated by a higher surface temperature. The data presented here are consistent with the rate-limiting step of the overall NO reduction process being the surface recombination of atomic nitrogen atoms resulting from fast dissociation of NO adsorbed molecules. Temperature-programmed desorption and CO titration experiments were also performed after the isothermal kinetic runs in order to estimate the surface coverages of the reactants during the steady-state reactions. The NO+CO conversion rate was found to be directly proportional to the coverage of atomic oxygen on the surface. The relation between reaction rates and nitrogen coverages, however, proved to be much more complex; an inverse correlation was in fact seen in most cases between those two parameters. The build-up of a critical coverage of atomic nitrogen was found to be necessary to trigger the nitrogen recombination step to N<SUB>2</SUB>. This critical coverage of strongly held nitrogen was determined to not depend in any significant way on the composition of the beam, but to decrease with reaction temperature in all cases

Surface intermediates during the catalytic reduction of NO on rhodium catalysts: a kinetic inference

Isothermal kinetic studies on the reduction of NO by CO on Rh(1 1 1) single-crystal surfaces, performed under vacuum by using collimated effusive molecular beams, have provided information on the coverages and nature of the surface intermediates involved in that reaction under catalytic conditions. Three major conclusions were reached. First, the optimum rate of reaction is achieved when the steady-state coverages of NO and CO on the surface reach the 1:1 stoichiometric ratio. The surface coverages are controlled by a synergistic balance between the composition of the gas and the surface temperature: higher temperatures tend to require higher CO:NO ratios. Second, under optimum conditions the surface of the catalyst is mostly covered with atomic nitrogen. This nitrogen appears to cluster in islands, and further conversion to molecular nitrogen takes place preferentially at their periphery. Finally, the formation of molecular nitrogen under catalytic conditions is likely to involve the formation of a N-NO intermediate. Evidence for these conclusions is provided

Selective production of methoxyphenols from dihydroxybenzenes on alkali metal ion-loaded MgO

Selective O-methylation of dihydroxybenzenes (DHBs; catechol, resorcinol, and hydroquinone) to methoxyphenols (MPs) was carried out with dimethylcarbonate on MgO and alkali metal ion (Li, K, and Cs)-loaded MgO between 523 and 603 K. Catalytic activity and product selectivity varied with respect to DHB substrates. Selectivity for O-methylated products increased with increasing basicity of alkali ions; however, K-MgO showed high and stable activity toward MPs. Selectivity for MPs obtained from three substrates increased in the following order: catechol &lt; resorcinol &lt; hydroquinone. The mode of interaction of substrates on the catalysts surface influenced reactivity and product selectivity. It is likely that the low reaction temperatures used (&lt;603 K) kinetically control and favor high MP selectivity from DHBs. Calcined and spent catalysts were characterized by XRD, surface area, SEM, thermal analysis, NMR, and XPS. XRD analysis revealed the formation of alkali oxide phases on alkali-loaded MgO. Crystallite size and surface area of the catalysts decreased after methylation reactions, except on K-MgO. TGA showed 40-60 wt% coke deposition on spent catalysts. TGA in N2 followed by air and 13C CP-MAS NMR measurements indicated the nature of deposited carbon to be molecular species, graphite, MgCO3 and polyaromatics. XPS revealed the nature and availability of active sites on the spent catalysts, as well as the same changes with reaction conditions and correlated with catalytic activity

NO+CO+O<SUB>2</SUB> reaction kinetics on Rh(111): a molecular beam study

Steady-state rates for the chemical conversion of NO+CO+O<SUB>2</SUB> mixtures on Rh(111) surfaces have been measured by using a molecular beam setup with mass spectrometry detection. The changes in the partial pressure of reactants (NO, CO, O<SUB>2</SUB>) and products (N<SUB>2</SUB> and CO<SUB>2</SUB>) have been used as a measure of the reaction rates for temperatures between 435 and 766 K and varying beam compositions around those observed in automobile exhausts. The addition of oxygen was found to inhibit the activity of the rhodium catalyst toward NO reduction in most cases, as expected. The reason for this behavior, however, was determined not to be the consumption of some of the CO in the mixture by the added O<SUB>2</SUB>, but rather a poisoning of the adsorption of CO by adsorbed atomic oxygen. In fact, oxygen addition to the NO+CO mixtures reduces not only the rates of N<SUB>2</SUB> production, but also those of CO<SUB>2</SUB> formation. Moreover, NO was proven to always compete favorably against O<SUB>2</SUB> for the consumption of CO. Optimum reaction rates for the production of both N<SUB>2</SUB> and CO<SUB>2</SUB> are reached at temperatures around 500-600 K and under conditions leading to stoichiometric coverages of all reactants. An interesting consequence of this is the fact that with CO-rich mixtures the addition of oxygen sometimes actually facilitates, not poisons, NO reduction, presumably because it helps in the removal of the excess CO from the surface. A synergy was observed in terms of reaction rate maxima between temperature and beam composition, CO-richer mixtures requiring higher temperatures to reach comparable reaction rates. This is explained by a decrease in CO surface coverage because of the increase in desorption rate with temperature, a trend that also explains the gradual increase in poisoning of the NO reduction activity of Rh by molecular oxygen with increasing temperature. Studies on the reaction between CO and O<SUB>2</SUB> were also carried out in order to isolate and identify the contributions of the surface oxygen deposited by dissociation of molecular oxygen and by NO to the production of CO<SUB>2</SUB>, and alternations between oxygen-rich and oxygen-lean beams were used to test cyclic processes as a way to better manage NO reduction under net oxidizing atmospheres